1. Field of the Invention
The present invention relates to a sub lithographic contact structure, in particular for a phase change memory cell, and a fabrication process thereof.
2. Description of the Related Art
As is known, phase change memory cells utilize a class of materials that have the unique property of being reversibly switchable from one phase to another with measurable distinct electrical properties associated with each phase. For example, these materials may change between an amorphous disordered phase and a crystalline, or polycrystalline, ordered phase. A material property that may change and provide a signature for each phase is the material resistivity, which is considerably different in the two states.
At present, alloys of elements of group VI of the periodic table, such as Te or Se, referred to as chalcogenides or chalcogenic materials, can advantageously be used in phase change cells. The currently most promising chalcogenide is formed by a Ge, Sb and Te alloy (Ge2Sb2Te5), which is currently widely used for storing information in overwritable disks.
In chalcogenides, the resistivity varies by two or more magnitude orders when the material passes from the amorphous phase (more resistive) to the polycrystalline phase (more conductive) and vice versa, as shown in FIG. 1. Furthermore, in the amorphous phase, resistivity strongly depends also on temperature, with variations of one magnitude order every 100° C., with a behavior similar to that of P-type semiconductor materials.
Phase change may be obtained by locally increasing the temperature, as shown in FIG. 2. Below 150° C. both phases are stable. Above 200° C. (temperature of start of nucleation, designated by Tx), fast nucleation of the crystallites takes place, and, if the material is kept at the crystallization temperature for a sufficient time (time t2), it changes its phase and becomes crystalline. To bring the chalcogenide back into the amorphous state, it is necessary to raise the temperature above the melting temperature Tm (approximately 600° C.) and then to cool the chalcogenide off rapidly (time t1).
From the electrical standpoint, it is possible to reach both critical temperatures, namely the crystallization and the melting temperatures, by causing a current to flow through a resistive element which heats the chalcogenic material by the Joule effect.
The basic structure of a PCM element 1 which operates according to the principles described above is shown in FIG. 3 and comprises a first electrode 2 (of resistive type, forming a heater); a programmable element 3 and a second electrode 5. The programmable element 3 is made of a chalcogenide and is normally in the polycrystalline state after processing. One part of the programmable element 3 is in direct contact with the first electrode 2 and forms the active portion affected by phase change, hereinafter referred to as the phase change portion 4.
In the PCM element 1 of FIG. 3, technological and electrical considerations impose that the contact area between the first electrode and the programmable element has small dimensions, so that, for the same current density, the writing operation may be carried out at the required local thermal energy with smaller current consumption.
Several proposals have been presented for reducing the contact area. For example, U.S. Pat. No. 6,294,452 discloses a process for forming a contact area of sublithographic dimensions, based on isotropically etching a polysilicon layer. The resulting sublithographic dimensions depend on the quality of the etching.
US 2001/0002046 discloses a process for forming an electrode of a chalcogenide switching device, wherein a spacer layer deposited in a lithographic opening is anisotropically etched and laterally defines an electrode. The resulting width of the electrode depends on the thickness of a spacer layer.
European patent application 01128461.9, filed on May 12, 2001, and entitled “Small Area Contact Region, High Efficiency Phase Change Memory Cell, And Manufacturing Method Thereof”, teaches forming the contact area as an intersection of two thin portions extending transversely with respect to one another and each of a sublithographic size. In order to form the thin portions, deposition of layers is adopted.
In all the indicated prior solutions, any variation in the electrode width L (FIG. 3), due for example to the process tolerances, affects, in a linear way, the contact area of the active region 4. Thus, the width L may have tolerances that are not acceptable as regards repeatability and uniformity of the cell characteristics.